What this calculator estimates and who it's for
A stormwater cistern captures roof runoff during rain events and releases it gradually for irrigation, non-potable reuse, or simply to reduce the volume and speed of water leaving your property. This calculator answers the practical sizing question every cistern owner eventually faces: given a specific rainfall event, how much water will actually reach the tank, and how many days will that water last before the next storm refills it? It also flags when a single storm is likely to overwhelm your cistern's capacity and spill to overflow.
It's built for homeowners, landscapers, and small-scale stormwater managers sizing a rain cistern or evaluating whether an existing tank is adequately matched to their roof area and water usage. It isn't a substitute for a certified stormwater management plan on commercial sites, which typically must account for local detention requirements, soil infiltration rates, and regulatory design storms beyond what this simplified model covers.
The formulas and what each variable means
Runoff Volume (gal) = Roof Area (sq ft) × Rainfall (in) × 0.623
The constant 0.623 converts one inch of rain falling on one square foot of roof into gallons (1 sq ft × 1 in = 1/12 cubic foot = 7.48 gallons × (1/12) ≈ 0.623 gallons) — it's a standard rainwater-harvesting conversion. From there: Captured Volume = Runoff Volume × Capture Efficiency, where Capture Efficiency accounts for losses from gutter overflow, debris screens, and first-flush diversion (85% is a common real-world estimate). Buffer Days = Captured Volume ÷ Daily Drawdown tells you how many days of typical water use the captured volume supports. Overflow Risk = max(0, (Runoff Volume − Cistern Capacity) ÷ Runoff Volume) × 100 estimates what percentage of a single event's runoff would spill past a full tank. The calculator compares that computed overflow risk against your entered "Overflow Percent" — your acceptable overflow tolerance — and flags whether the event's risk exceeds what you consider acceptable.
Worked example
A 2,000 sq ft roof during a 1-inch rain event: Runoff = 2,000 × 1 × 0.623 = 1,246 gallons. With 85% capture efficiency: Captured = 1,246 × 0.85 = 1,059 gallons. At a 200 gallon/day drawdown: Buffer = 1,059 ÷ 200 = 5.3 days. Against a 1,000-gallon cistern: Overflow Risk = max(0, (1,246 − 1,000) ÷ 1,246) × 100 = 19.7% — since that exceeds a 10% overflow tolerance, the calculator now flags that this event exceeds the acceptable tolerance and suggests a larger cistern or added drawdown. Entering these exact values reproduces every one of these figures in the calculator above.
Common mistakes and how to interpret the result
- Using total roof area instead of the area draining to this cistern. If only part of your roof drains to the downspout feeding this cistern, use that portion's area, not the whole building's footprint.
- Overestimating capture efficiency. Real systems lose water to gutter overflow during intense storms, leaf screens, and first-flush diverters that intentionally discard the initial dirty runoff — 80-90% is realistic for a well-maintained system, not 100%.
- Sizing for an average storm instead of a design storm. A cistern that handles typical rainfall comfortably can still overflow during a single heavy event — check your overflow risk at a larger, less frequent storm size (2-3 inches) if managing peak runoff matters to you.
- Ignoring drawdown between storms. A large cistern with slow drawdown provides little benefit if it's already full when the next storm arrives — buffer days should be compared against your local typical time between rain events.